DESCRIPTION: Integration, the insertion of the retroviral viral genome into the host genome by the viral integrase is an obligate part of the retrovirus life cycle. The long terminal repeat (LTR) portions of the viral DNA termini serve as substrates for the integration reaction. The first enzymatic step of integration, 3'OH processing, is the removal of the two terminal nucleotides at both 3' ends of the viral LTR DNA , mediated by IN using water as the nucleophile. The proposed research will define the molecular and biochemical basis for substrate specificity among various integrases for 3'OH processing. In light of the small amount of information available on the molecular basis of LTR recognition during 3' OH processing, a preliminary cross-comparative analysis of the substrate specificity of several species of integrases was performed. These studies have identified nucleotides for HTLV-II proximal to the conserved CA dinucleotide that are essential for 3'OH processing and contribute to substrate specificity. The first year will continue work concerning substrate specificity during 3' OH processing using a new fluorescent assay. Nucleotides involved in substrate specificity for HIV-1, murine leukemia virus, HTLV-I, and HTLV-II will be determined. The results should allow the development of a model of the active site of integrase.